The early Drosophila embryo is an ideal model to understand the transcriptional regulation of well-defined patterns of gene expression in a developing organism. In this system, snapshots of transcription measurements obtained by RNA FISH on fixed samples cannot provide the temporal resolution needed to distinguish spatial heterogeneity from inherent noise. Here, we used the MS2-MCP system to visualize in living embryos nascent transcripts expressed from the canonical hunchback (hb) promoter under the control of Bicoid (Bcd). The hb-MS2 reporter is expressed as synchronously as endogenous hb in the anterior half of the embryo, but unlike hb it is also active in the posterior, though more heterogeneously and more transiently than in the anterior. The length and intensity of active transcription periods in the anterior are strongly reduced in absence of Bcd, whereas posterior ones are mostly Bcd independent. This posterior noisy signal decreases progressively through nuclear divisions, so that the MS2 reporter expression mimics the known anterior hb pattern at cellularization. We propose that the establishment of the hb pattern relies on Bcd-dependent lengthening of transcriptional activity periods in the anterior and may require two distinct repression mechanisms in the posterior.

SUMMARYIt is widely accepted that morphogenetic gradients determine cell identity by concentration-dependent activation of target genes. How precise is each step in the gene expression process that acts downstream of morphogens, however, remains unclear. The Bicoid morphogen is a transcription factor directly activating its target genes and provides thus a simple system to address this issue in a quantitative manner. Recent studies indicate that the Bicoid gradient is precisely established in Drosophila embryos after eight nuclear divisions (cycle 9) and that target protein expression is specified five divisions later (cycle 14), with a precision that corresponds to a relative difference of Bicoid concentration of 10%. To understand how such precision was achieved, we directly analyzed nascent transcripts of the hunchback target gene at their site of synthesis. Most anterior nuclei in cycle 11 interphasic embryos exhibit efficient biallelic transcription of hunchback and this synchronous expression is specified within a 10% difference of Bicoid concentration. The fast diffusion of Bcd-EGFP (7.7 m 2 /s) that we captured by fluorescent correlation spectroscopy in the nucleus is consistent with this robust expression at cycle 11. However, given the interruption of transcription during mitosis, it remains too slow to be consistent with precise de novo reading of Bicoid concentration at each interphase, suggesting the existence of a memorization process that recalls this information from earlier cycles. The two anterior maternal morphogens, Bicoid and Hunchback, contribute differently to this early response: whereas Bicoid provides dose-dependent positional information along the axis, maternal Hunchback is required for the synchrony of the response and is therefore likely to be involved in this memorization process.